Where nuclear reactors, more particularly pressurized-water nuclear reactors, are concerned, it may be necessary to join large-size components, such as pipes of the primary circuit, by welding. In particular, such operations of joining together by welding may have to be carried out after a certain reactor operating time, during which the pipes of the primary circuit have acquired a relatively high radioactivity.
There has, for example, been a recent proposal to increase the service life of a nuclear power station by replacing with new steam generators the worn steam generators of a pressurized-water nuclear reactor, the tube bundle of which has become defective during long-term use.
To carry out this operation to replace a steam generator as a whole, it is necessary, in particular, for the primary-water inlet and outlet connections for the steam generator to be joined by welding to the corresponding pipes of the primary circuit which are held on standby in the reactor building.
In order to remove the worn steam generator, it is necessary to cut some parts of the primary pipes in the vicinity of the connections leaving the water box. After cutting, the ends of the pipes are remachined, and the connections of the new generator intended to replace the worn generator are designed to fit the pipes of the primary circuit which have been severed and remachined. The connections and pipes which have a large thickness, which may be of the order of 0.10 meters, and a larger diameter, which can rise to a value near a meter, then have to be joined together by welding. The machining of the cut end of the pipe and of the connection makes it possible to form between them a chamfer of specific shape open towards the outside an d limited towards the inside by two corresponding squeeze parts which are fixed in place by manual welding at the start of the joining operation.
The chamfer is then filled with build-up metal is successive passes by means of orbital TIG welding.
In the construction of a nuclear power station, the steam generators can be connected to the primary circuit on site without serious difficulties, the operator supervising and controlling the orbital welding device at the very locations where the welding operation is carried out, i.e., in the casemate of the steam generator, thus having direct control of the welding operation. The quality of the welding joint, which is subjected to an extremely stringent control (zero defect), therefore requires highly qualified operators whose concentration must be maintained during the entire execution of the welding pass.
To fill the chamfer completely, there must be a large number of welding beads, each laid all round the chamfer during an orbital welding pass. In order to join a generator connection and a pipe of the primary circuit, at present 185 welding beads must be deposited during the same number of orbital welding passes.
When a steam generator of a reactor which has already been in operation is changed, the primary connections are highly irradiated, and it is no longer possible to monitor and control the orbital welding operation from a point near the primary pipe.
There are known automatic orbital TIG welding processes which could theoretically make it possible to join the connection and the pipe of the primary circuit together by welding, without an operator being present in the casemate of the steam generator. However, in such automatic welding processes, a large number of parameters for the welding operation have to be preset in order to ensure that successive passes are executed satisfactorily. But during an operation lasting a very long time, some parameters, such as the position of the electrode in relation to the bath and in relation to the chamfer, still have to be monitored and adjusted continuously according to an examination of the welding zone. Theoretically, this continuous check could be carried out automatically, but the control of the process would then be too complex and would make it necessary to use extremely sophisticated and extremely expensive equipment.
There has therefore been a proposal for an operator to carry out televisual monitoring of the welding zone, this operator making the desired corrections of a certain number of parameters remotely by means of a remote-control unit.
Such a semi-automatic welding process cannot be used satisfactorily to join the connections of a steam generator to the corresponding pipes of the primary circuit of the reactor, because, where the quality of the weld is concerned, it is largely based on a continuous adjustment of parameters carried out by an operator from a television picture of the welding zone.
In fact, it is still possible for the operation to be carried out so that the welded joint does not have any defect, as long as the operator is actually present in the welding zone and can have direct access to the means of adjusting certain welding parameters. In contrast, when monitoring is carried out via a television image, analyzing the zone during welding and making a decision as to the parameters to be adjusted demand considerable effort and special training of the operator.
Moreover, if a joining chamfer of a steamgenerator connection is filled with build-up metal, the welding parameters relating to each of the passes will be different from the parameters of the pass executed immediately beforehand or from the parameters of the pass executed immediately thereafter. The adjustment intervals for the parameters controlled by the operator are also essentially variable from one pass to another. It therefore becomes very difficult, if not impossible, to make the desired adjustments at the start of a welding pass and control the adjustable parameters satisfactorily during the welding pass.
Although a certain methodology for welding large-size components, especially pipelines, by depositing metal in a chamfer, has been developed, the amount of information necessary to carry out the welding operation successfully in nevertheless too great to be used effectively by an operator monitoring the execution of the welding operation continuously.
When pipelines, such as a steam-generator connection and the corresponding pipe of the primary circuit of a reactor, are joined together, the build-up metal is deposited in successive layers, each of the layers consisting of welding beads, each formed during a pass all round the annular chamfer and placed next to one another. Each pass, depending on the position of the corresponding bead in the chamfer, can be linked to a predefined type which makes it possible to determine the welding parameters. These parameters therefore depend particularly on the type of pass and layer to which the bead being deposited belongs. It is clear that the mass of information necessary for carrying out the welding operation successfully is very great in an operation requiring no less than 185 welding beads.